1. Field of the Invention
The present invention relates to a printer, and more particularly to an ink supply construction of a stencil printer.
2. Description of the Prior Art
As an ink supply construction of a stencil printer, there has been proposed by Japanese Patent Application 53-128043 (Laid-open Publication 55- 55889) by the same assignee as that of the present application such an ink supply construction of a stencil printer that comprises a frame including a pair of mutually opposing side plates, an ink supply roller supported by the frame between the pair of side plates to rotate about a central axis thereof with an outer circumferential surface thereof slidably contacting an inner circumferential surface of the printing drum of the printer, and an ink supply control member supported by the frame between the pair of side plates to extend along a generatrix of the ink supply roller so as to define a wedge-shaped ink holding space converging to an ink metering clearance together with an outer circumferential surface of the ink supply roller, so that the ink supplied to the ink holding space is formed into a lump of ink rotating in the ink holding space llike a vortex conforming to the moving direction of the outer circumferential surface of the ink supply roller, with the ink of the lump being transferred through the ink metering clearance according to the rotation of the ink supply roller as adhered to the outer circumferential surface thereof.
Further, it has been proposed by Japanese Patent Application 58-113850 (Laid-open Publication 60-4062) filed by the same assignee as that of the present application to dispose an ink driving rod member in the ink holding space to extend along a generatrix of the ink supply roller and the ink supply control member and to rotate around a central axis thereof in synchronization with the rotation of the ink supply roller in a rotational direction opposite to that of the ink supply roller, in order to intensify the vortex-like rotation of the lump of ink formed in the ink holding space.
Still further, it has been proposed by Japanese Utility Model Application 59-107284 (Laid-open Publication 61-22655) by the same assignee as that of the present application to form the ink driving rod member such that it has a larger cross section in axially opposite end portions thereof than in an axially central portion thereof, in view of a phenomenon that a lump of ink rotating like a vortex in the ink holding space tends to have a larger cross section in the axially opposite end portions than in the axially central portion due to the fact that the axially opposite end portions are more decelerated than the axially central portion by a frictional contact with the pair of side plates.
In each of the above-mentioned formerly proposed ink supply constructions, the lump of ink rotating in the ink holding space like a vortex is formed by the ink pressurized by an ink pump and discharged from a plurality of ink outlet ports disposed above the ink holding space as distributed therearound, with the size of its cross section being maintained within a predetermined range by the ink pump being controlled by a control means in response to an output of an ink sensor for detecting the cross sectional size of the lump of ink. In more detail, when the cross sectional size of the lump of ink decreases below a predetermined value according to a discharge of the ink from the ink holding space along with the rotation of the ink supply roller accompanying the rotation of the printing drum during a printing, the reduction of the cross sectional size is detected by the ink sensor of a contacting or non-contacting type, whereupon the ink pump is operated by the control means so as to supplement ink to the ink holding space, and when an increase of the cross sectional size of the lump of ink is detected by the ink sensor after a lapse of time of the supplement of ink, the operation of the ink pump is stopped by the control means. By such an intermittent operation of the ink pump by the control means accompanied by the ink sensor, the cross sectional size of the lump of ink in the ink holding space is maintained to be substantially constant during the operation of the ink supply construction.
However, since the supply of ink to the ink holding space is made by a plurality of ink outlet ports arranged as spaced along the ink holding space, during a period in which the ink is being discharged from the ink outlet ports and a little while after the stop of the supply of ink the cross sections of the lump of ink at and around the portions thereof corresponding to the ink outlet ports are larger than those of the other portions, so that, even by an improvement available by the above-mentioned Japanese Patent Application 55-113850 and Japanese Utility Model Application 59-107284, the cross sectional size of the lump of ink is still not uniform along its length, thereby threatening a generation of non uniform thickness of the printed image in the direction of width of the paper. Further, since the ink sensor detects the cross sectional size of only a portion of the lump of ink along the length thereof, there inevitably occurs a discrepancy between the amount of ink supplied to the ink holding space and the amount of ink detected by the ink sensor, such a discrepancy becoming greater as the speed of supply of ink, i.e., the speed of consumption of ink, increases.
Since the wedge-shaped cross section of the ink holding space defined between the outer circumferential surface of the ink supply roller and the ink supply control member is opened relatively widely upward, with its lower edge defined by the outer circumferential surface of the ink supply roller swiftly approaching to being horizontal as being more distant from the tip end of the wedge, in the conventional ink supply construction the outer circumferential surface of the ink supply roller defining the lower edge of the wedge-shaped ink holding space or the lower wall thereof has substantially no function of preventing a lateral expansion of the cross sectional shape of the lump of ink so as to restrict an excessive local growth of the lump of ink not even along the length thereof.